1. Field of Invention
This invention generally relates to load indicating fasteners, and more particularly, to a low profile load indicating and energy-storing device adapted to visually indicate the load in a fastener.
2. Description of the Related Art
Fasteners are used in a wide variety of applications, such as motors, railroad tracks, flange assemblies, petrochemical lines, foundations, mills, drag lines, power turbines and studs on cranes and tractors. As is known in the use of such fasteners, as force is applied to a portion of the fastener, e.g. a head of a bolt or the like, the fastener experiences a strain described as the fastener load. As the fastener is tightened, this load increases to a maximum break point, where the fastener breaks or its integrity is otherwise compromised. Therefore, it is desirable that applied fasteners should be properly tightened to design load levels in order to ensure that secure joints are achieved without compromising the fastener viability. In many applications, however, achieving the proper fastener tightness and maintaining this tightness once the system is placed in service is problematic. For various applications, optimal loads are known and/or are obtainable, but currently available methods and apparatus do not adequately enable reliable and repeatable determinations thereof.
During use, fasteners typically experience a loss of tension e.g. tightness, due to, for example, a variety of in-service occurrences including: relaxation (thread embedment), vibration loosening, compressive deformation in the joint or flange, temperature expansion or contraction, etc. The loss of tension that results from these occurrences can cause premature wear in the assembly, leakage (in applications where the fastener is used for sealing), or critical joint failure due to excessively high loads on other members of the assembly. Such potential failures are catastrophic in systems where premature wear, leakage or joint failure may result in loss of life.
An apparatus and method is therefore needed which permits the accurate tightening of a fastener to optimal load levels and which permits the determination of the existing fastener load status.
It is well known that indicated tensile strain gives a true representation of the load induced in a fastener. Various prior art tensile strain indicators may concentrate on the tensile strain of the individual members of the fastener, such as for example, the fastener washer. One such indicator is described in U.S. Pat. No. 6,250,863, issued to Kamentser et al., which purportedly discloses a washer having a plurality of strain gauges integrated into the body of the washer. The Kamentser patent discloses two sets of strain gauges, at least one of which is positioned on a portion of the washer subjected to axial force, and at least one of which is positioned on a portion of the washer not subjected to axial force. The strain gauges are connected into a common bridge circuit which purportedly provides a signal indicating the axial stress applied to the washer body.
Several problems, however, are associated with the fasteners described in the Kamentser patent. For example, in most instances it is cost prohibitive to integrate the electrical measuring devices into the body of each individual washer. Additionally, the integrated instrumentation, which are placed in the body of the washer, compromises the overall integrity of the fastener and is thus not suitable for applications using fasteners in a rugged environment. Thus, a need exists for a device to measure tensile strain, is not cost prohibitive, and which additionally does not compromise the overall fastener integrity when used in a rugged environment.
Other prior art systems avoid the problems inherent in using integrated electrical components and instrumentation by using mechanical load indicators. For example, UK Pat. Number GB-2-179-459-A, issued to Ceney, discloses an externally mounted mechanism for indicating the tightness of a fastener. This system includes a pin positioned in the bore of the fastener that extends out of the fastener end. Upon extension of the bolt, the pin applies pressure to fulcrumed levers positioned perpendicular to the axis of the bolt. The levers, which are acted upon by a compression spring, are then visible through a window cover for visually inspecting washer load levels.
Due to the complex arrangement of the levers, and position of the indicator window inherent in the design of this system, the indicator components typically must be positioned on the outside of the bolt. Since the indicator components are rather bulky relative to the washer, the use of the Ceney system is often not possible in fastener environments having space constraints. In cases where it is possible to use such a configuration, the elements of the instrument may be susceptible to outside forces and damage. Upon damage, no convenient method exists to verify whether the unit may still be calibrated.
Therefore, a need exist for a load indicating system which conforms to the space limitations of a fastener environment, and which includes means for readily identifying when the load indicator is not valid or the fastener is not at its desired load level.
It should be further noted that one of the main indicators of efficient clamp retention in a joint is the amount of elastic energy the bolt and other joint members can absorb. Regardless of the tightening technique used, when a bolt in a bolted joint is tightened, the bolt stretches elastically and stores energy. In this manner, the bolt acts as a spring and the stored energy facilitates the holding of the joint together at a specified load level.
To ensure that a fastener includes the highest amount of elastic energy, fastener designers typically focus on the grip length of the bolt used in the system. It is well known that the longer the grip length of a bolt, the more the energy the bolt will store in the fastener system. Consequently, a longer bolt having a longer grip length, will be able to store more elastic energy than a shorter bolt having a shorter grip length. Thus, the increased grip length of a longer bolt makes using the longer bolt in a fastener more preferable than using the shorter bolt. This is true because, the increased energy stored in the longer bolt is advantageous in that it enhances the integrity of the joint making the joint more tolerable to loosening or failure due to in-service loading.
In many fastener systems, however, the use of the longer bolt is prohibited by space limitations, requiring the system to use a shorter bolt. As noted, because of the shorter grip length, the shorter bolt stores less elastic energy than the longer bolt, which makes the fastener more susceptible to loosing or failure. It is well known, however, that the grip length of the shorter bolt can be made longer, through the use of washers or sleeves which cause the shorter bolt to elastically stretch further and store more energy.
Unfortunately, the bolt's shorter length puts an upper limit on the number of washers which may be used to increase the shorter bolt's grip length. This, in turn, means that, in general, a shorter bolt may only be made to store a limited amount of elastic energy to aid in holding the joint together. Therefore, where, as in many instances, there are space constraints requiring the use of the shorter bolt, a fastening system may be used which results in poorly designed joints, since the shorter bolt may store an inadequate amount of energy. This poor grip length bolt diameter-to-length ratio may inevitably lead to joint loosening or failure.
A fastener system is therefore desired which would allow use of a shorter bolt while not compromising the amount of energy stored in the fastener when a longer bolt is used. Such a fastener system may include the storage of additional elastic energy above that already stored in the bolt, and may additionally increase the effective grip length of the shorter bolt improving the fastener clamp retention property.
Consequently, presently known fasteners employing load indicators remain inadequate, particularly since these fasteners typically incorporate integrated electrical components, complex designs and/or are subject to loss of effectiveness do to loss of calibration or elastic energy over extended use. A need, therefore, exists for a load indicating fastener system which avoids the problems inherent in the prior art while providing an accurate reading of the tensile strain, e.g., load, being placed on the fastener system, and which stores additional energy allowing for the effective use of shorter bolts in a particular fastener system.